Changing camera background image proxy for easier viewing
Are you using Blender 2.83?
I would say you are luckier than me because when I change the proxy resolution I don't have any change at all in the 3D viewport :/
I am not sure if a manual reload of the clip is required - or a full project file reload maybe?
In any cases a pink image means the expected media is not found at the current time. This can be due to:

The current time your scene is using is out of the media range (it can be after of before, even if there is a strange behavior for before in the way that after some time jog the first frame of the media is displayed instead of the pink fill). By moving the current time you should be able to reach the media range (which should not have changed btw).
Or the proxy has not been generated, for some reason...

This is not really an answer but...
At this point I am wondering if this property is working correctly on this version...